1. Field of the Invention
This invention generally relates to digital-to-analog converters (DACs) and, more particularly, a system and method for calibrating a DAC.
2. Description of the Related Art
A current-steering uses DAC uses a number of current sources (CS) whose currents are switched to a summing node. The current sum is an analog value that is intended to represent a digital value input to the DAC. In a differential current-steering DAC, separate currents for each CS are switched to positive and negative summing nodes.
A thermometer-coded DAC contains a current-source segment for each possible value of DAC output. An 8-bit thermometer DAC would have 255 segments, and a 16-bit thermometer DAC would have 65,535 segments. This is one of the fastest digital-to-analog conversion methods but suffers of low precision due to the accuracy requirements for each current source. To achieve higher accuracy a technique called “segmentation” is often used, i.e. the current sources are divided in two “segments”, the least-significant bits (LSBs) that are typically binary scaled, and the most-significant thermometer-coded bits that are typically twice the largest LSB. In this way, a compromise is obtained between precision (by the use of the thermometer-coded principle) and number of current sources (by the use of the binary-weighted principle). In order to achieve the desired output resolution, the current sources must have exact ratios. If their ratios are not exact (e.g. the current sources are “mismatched”), the DAC suffers from a lower effective resolution than nominal.
There are few methods known to mitigate the issues associated with DAC calibration. The current sources can be increased in size to improve matching. However, this solution requires that the DAC device becomes bigger (and more expensive), and the internal signals must travel a longer distance, thus limiting the maximum speed. Calibration switches can be added to equalize the size of the current sources. Again however, this solution results in a larger DAC device, and the calibration switches add extra complexity. Analog calibration can also be implemented by using capacitors to store a calibration charge. As in the solutions above, the DAC device becomes bigger due to the extra area for the capacitors and switches. Moreover, the analog complexity is also increased. Calibration can also be performed digitally. However, this solution requires the use of an analog-to-digital converter (ADC), which must have the same analog input dynamic range as the DAC itself, which results in complicated analog circuitry.
It would be advantageous if a DAC could be calibrated using a low dynamic range ADC by comparing all the current sources to a single reference.
It would be advantageous if the DAC current sources could be compared to the single reference by recording the differences between adjacent current sources.